1. Field of the Invention
The invention relates to an inverter controlling method, and more particularly, to a method for automatically correcting variations in output voltage of inverter due to variations in power supply voltage.
2. Description of the Prior Art
Such an inverter controlling method used so far is shown FIG. 1, in which reference numeral 1 illustrates a commercial AC power supply, 2 a power rectifier, 3 a smoothing capacitor, 4 a power inverter, 5 an AC motor as a load, 6 a frequency setter for the inverter, 7 a soft start circuit, 8 a frequency controlling circuit, 9 a comparator for comparing the voltage instruction from the soft start circuit 7 with the output voltage of the later-mentioned inverter-output voltage detecting circuit 13 as set forth later, 10 an amplifier, 11 an inverter-output voltage controlling circuit, 12 a base drive circuit for the power inverter, and 13 an inverter-output voltage detecting circuit.
The mode of the operation of the prior art inverter-controlling method shown in FIG. 1 will be described below. AC power from the commercial power supply 1 is converted to DC power in the power rectifier 2, smoothed in the smoothing capacitor 3 and input into the power inverter 4. The power inverter 4 which, for example, consists of a transistor and other elements, reconverts the DC power supplied in response to the switching of the transistor into AC power having variable frequency and variable voltage, and thus causes the AC motor 5 as a load to run at variable speed. The control circuit for operating the power inverter 4 as a switch operates as follows. The analog frequency instruction signal provided by the frequency setter 6 is input into the soft start circuit 7 and there converted into two series of signals, one for controlling the output frequency of the inverter 4 and the other for controlling the output voltage. The frequency controlling signal series is supplied to the base drive circuit 12 through the frequency controlling circuit 8, and actuates the base drive circuit 12 to control the output frequency of the power inverter 4. The voltage controlling signal series is input into the comparator 9, and thereby is compared with the output signal of the inverter-output voltage detecting circuit 13, and amplified by the amplifier 10, then the thus-obtained signal is input into the base drive circuit 12 through a output voltage control circuit 11. The output signal of the frequency control circuit 8 is also input to the base drive circuit 12. Then the base drive circuit 12 is actuated to control the output voltage of the inverter 4.
The prior art inverter controlling circuitry of the construction above-described allows only slight variation in the output voltage due to the variation in power supply voltage by virtue of the effect of the closed-loop control on the output voltage. It, however, is defective in respect that the constructed closed-loop exerts inverse influence of impaired responsiveness of the output voltage control, and it employs complicated overall circuit.